stephen at kormilo.ca
Tue Feb 10 11:31:31 EST 2004
At 01:57 AM 10/02/04 -0500, quattro-request at audifans.com wrote:
>Date: Tue, 10 Feb 2004 01:57:48 -0500
>From: Huw Powell <audi at humanspeakers.com>
>Subject: Re: Electrical schoolwork...
>To: SJ <syljay at optonline.net>
>Cc: quattro at audifans.com
>Message-ID: <402880EC.1090505 at humanspeakers.com>
>Content-Type: text/plain; charset=us-ascii; format=flowed
>I prefer this answer to Ti's current based calculation.
Well, notwithstanding your preference, Ti's answer is correct. A
resistance of ~700 k-ohms will result in a _very_ dim LED ;-)
The brightness of the LED depends on the _current_ through it, and, just
like other types of diodes, the voltage across a forward-biased LED remains
nearly constant, typically at around 2.5V at normal operating currents,
typically ~20 mA. So a resistance in the 500 ohm range will do the
job. Exact resistance values are far from being critical, unless they
result in currents that exceed the maximum LED current rating, typically
~50 mA or so,
The LED resistance figure given in the original poster's query is totally
meaningless and was probably measured with an ohmmeter.
>I would recommend testing your choice "out in the open" before
>rebuilding a fistful of switches, etc., to make sure you got it right.
I would recommend this approach to determine the desired brightness
level. Use a fixed value resistor of ~330 ohms in series with a ~500 ohm
variable resistor to 'play' with the brightness level. Don't forget that
LEDs are polarity sensitive. They will not light when
reverse-biased. Connect the LED Anode to +'ve through the resistor and the
LED Cathode to Ground.
> >>From: "Louis-Alain RICHARD" <laraa at sympatico.ca>
> >>Resistance value of the LEDs: 150 000 ohms
> >>Ideal voltage at the LEDs: between 2.5V and 3V.
> >>Electrical voltage at the switches: 13.6V, permanent, switched by the
> > ***** For 3 volts
> > x/150k = 12v/3v
> > x = (12v/3v)*150K
> > x = 600K
> > for 2.5 volts
> > x/150k = 12v/2.5v
> > x = (12v/2.5v)*150k
> > x = 720K
> > You dont need an exact value. Anything close will do.
Former (now retired) Electronics Technology Instructor.
Silver 2002 Audi A4 3.0QM
Now departed: Silver '98.5 Audi A4 2.8QM
Black '85 Mazda Rx7 GSL-SE
email: skormilo at rrc.mb.ca
stephen at kormilo.ca
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